Afocal optical system and multibeam recording apparatus comprising the same

ABSTRACT

An afocal optical system is formed by a paraboroid mirror and an optical element having a stereographic projection characteristics which is defined by the following equation: 
     
         hi&#39;=2·f·tan(θi/2) 
    
     where hi&#39; is a height of a light beam, leaving the optical element, taken from the optical axis or a heigh of an image taken from the optical axis, f is a focal length of the optical element and θi is an angle of incidence with respect to the optical element. The focal point of the optical element coincide with that of the first paraboroid mirror. Thus, a compact afocal optical system which satisfies hi&#39;=m·hi is manufactured at low costs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an afocal optical system in which focalpoints of optical elements each having a finite focal length coincidewith each other at a predetermined point. The present invention alsorelates to a multibeam recording apparatus comprising the afocal opticalsystem.

2. Description of the Background Art

FIG. 1 is a diagram of a conventional afocal optical system which isknown in the art as a Keplerian type beam expander. The beam expander iscomprised of two positive power lenses L31 and L32 which are spaced awayfrom each other by a distance (f31+f32), where f31 is a focal length ofthe lens L31 and f32 is a focal length of the lens L32. The focal pointof the lens L31 and the focal point of the lens L32 coincide with eachother at a predetermined point A. Hence, a light beam LB1 parallel tothe optical axis Z entering the beam expander would be converted into alight beam LB3 which leaves the beam expander parallel to the opticalaxis Z, the light beam LB3 satisfying: ##EQU1## where hi is a height ofthe incident light beam LB1 taken from the optical axis Z; hi' is aheight of the leaving light beam LB3 taken from the optical axis Z; andm is a magnification of the beam expander.

FIG. 2 is a diagram showing other example of a conventional afocaloptical system and illustrates the relationship between an object 202and an image 204. The afocal optical system is comprised of a lens L41having a focal length f41 and a lens L42 having a focal length f42,which are disposed a distance (f41+f42) away from each other. The imageof the object 202 placed the distance f41 in front of (on the left sideof) the lens L41 is obtained at a point the distance f42 behind (on theright side of) the lens L42.

The beam expander of FIG. 1 needs to comprise a larger lens L32 if theincrease in diameter of light beam LB3 is desired. Likewise, the bothside telecentric optical system of FIG. 2, which is telecentric on boththe image and the object sides, needs to comprise a larger lens L42 if alarger image 204 is desired.

When reduction in size of these optical systems (shortened optical path)is desired, one of the approaches to attain is to shorten the focallength f32 of the lens L32, for example. However, such would result inreduction in the F-number of the optical system, which in turn wouldrequire an increased number of lenses to be used to achieve an opticalsystem which is capable of carrying out the same optical performance asthe optical system of FIG. 1. Thus, although the optical path isshortened, the number of lenses which form the optical system isincreased, and hence, the manufacturing cost and the weight of theoptical system are increased.

Conversely, if the optical system is formed by less lenses to placepriority on the number of the lenses to form the optical system, theoptical performances of the optical system would be deteriorated,creating various aberrations. Since the optical system can no longersatisfy the relation hi'=m·hi due to the aberrations, the optical systemis not reliable enough to be applied to an optical apparatus such as abeam expander and a multibeam recording apparatus.

When the optical system can not satisfy the relation hi'=m·hi, chancesare that even if the light beam LB1 impinging upon the afocal opticalsystem is parallel to the optical axis Z, the light beam LB3 leaving theoptical system is not parallel to the optical axis Z. In such likelycase, if the optical system is used in an apparatus which requires atelecentric characteristic especially on the imaging side, an imagewould be distorted.

FIG. 3 is a diagram of a conventional multibeam recording apparatus. InFIG. 3, the multibeam recording apparatus comprises a plurality of lightsource parts which are arranged at equal intervals (only one lightsource part 12 is shown in FIG. 3), a reduction optical system 200 whichis formed by lenses L20 and L21, a zoom lens 32 which is formed bylenses L22 to L24, and an afocal optical system 34 which is formed bylenses L25 and L26.

The light source part 12 includes a semiconductor laser 14. A laser beamfrom the semiconductor laser 14 is collimated by a collimating lens 16,and then pass through an aperture 18 to be allowed to the reductionoptical system 200 parallel to the optical axis Z. The reduction opticalsystem 200 has the same structure as that of the conventional afocaloptical system of FIG. 2. That is, as shown in FIG. 3, the rear focalpoint of the lens L20 coincides with the front focal point of the lensL21, and therefore, the reduction optical system 200 is an afocaloptical system. The laser beams from the reduction optical system 200are magnified at a proper magnification by the zoom lens 32, focused bythe afocal optical system 34 at the focal plane FP3 of the afocaloptical system 34, and irradiated onto a recording surface RS which isdisposed at the focal plane FP3 of the afocal optical system 34. Sinceprincipal rays of the laser beams are each perpendicular to the focalplane FP3, a magnification does not change even when a distance betweenthe focal plane FP3 and the recording surface RS is changed. Thus,highly accurate image drawing is attainable.

Laser beams from the other light source parts which are not shown areirradiated onto the recording surface RS in a similar manner so that aplurality of beam spots are formed at the same time on the recordingsurface RS.

Constructed as above, the conventional multibeam recording apparatusneeds a larger lens in order to increase the number of the beam spotswhich are formed on the recording surface RS at one time, i.e., thenumber of the channels. As can be understood from FIG. 3, to obtain morechannels, more light source parts 12 disposed in a directionperpendicular to the optical axis Z are necessary, and therefore, thelens L20 must be enlarged accordingly at the expense of deterioratedaberration at the lens L20 and increased costs for manufacturing thelens L20.

On the other hand, to obtain a smaller multibeam recording apparatus byreducing the size of the optical system of FIG. 3, the focal length f0of the lens L20 and hence the optical path must be shortened. However,when the focal length f0 is reduced, the F-number of the optical systemwill become smaller. In such a case, an increased number of lenses mustbe used to ensure the same optical performance which are obtainable fromthe optical system of FIG. 3. As a result, although the optical path isshortened, the number of the lenses which form the optical system, andhence, the manufacturing costs and the weight of the optical system areincreased.

Conversely, if the optical system is formed by less lenses to placepriority on the number of the lenses to form the optical system, theoptical performances of the optical system would be deteriorated,creating various aberrations. Hence, although the light source parts 12are arranged equidistant from each other, spacings between adjacent beamspots which are irradiated through the optical system onto the recordingsurface RS, that is, the beam pitches, will become uneven or theconfigurations of the beam spots will be deformed. Further, since theprincipal rays of the laser beams striking the focal plane FP3 are notperpendicular to the focal plane FP3, with a change in a distancebetween the focal plane FP3 and the recording surface RS, themagnification of the optical system (beam pitches) will be changed. Aresult of this is degraded quality of a recorded image.

SUMMARY OF THE INVENTION

The present invention is directed to an afocal optical system having anoptical axis, comprising: a first paraboroid mirror, disposed on theoptical axis, having a finite focal length; and an optical element,disposed on the optical axis, having a stereographic projectioncharacteristics which is defined by the following equation:

    hi'=2·f·tan(θi/2)

where hi' is a height of a light beam, leaving the optical element,taken from the optical axis or a height of an image taken from theoptical axis, f is a focal length of the optical element and θi is anangle of incidence with respect to the optical element, the focal pointof the optical element substantially coinciding with that of the firstparaboroid mirror.

In another aspect of the present invention, an afocal optical systemcomprises: a spherical mirror, disposed on the optical axis, having afirst focal length; and an equisolidangle projection lens, disposed onthe optical axis, having an optical characteristics defined by thefollowing equation:

    hi'=2·f·sin(θi/2)

where hi' is a height of a light beam, leaving the equisolidangleprojection lens, taken from the optical axis or a height of an imagetaken from the optical axis, f is a focal length of the equisolidangleprojection lens and θi is an angle of incidence with respect to theequisolidangle projection lens, the focal point of the equisolidangleprojection lens substantially coinciding with that of the sphericalmirror.

The present invention is directed to a multibeam recording apparatus forrecording an image on a recording surface, comprising: a light sourceunit for emitting a plurality of light beams; and a reduction afocaloptical system for directing the light beams from the light source unittoward the recording surface, the reduction afocal optical system havingan optical axis, wherein the reduction afocal optical system comprises afirst paraboroid mirror, disposed on the optical axis, having a firstfinite focal length; and an optical element, disposed on the opticalaxis, having a stereographic projection characteristics which is definedby the following equation:

    hi'=2·f·tan(θi/2)

where hi' is a height of a light beam, leaving the optical element,taken from the optical axis or a height of an image taken from theoptical axis, f is a second finite focal length of the optical elementand θi is an angle of incidence with respect to the optical element; andwherein the focal point of the optical element substantially coincideswith that of the first paraboroid mirror.

In another aspect of the present invention, a multibeam recordingapparatus for recording an image on a recording surface comprises: alight source unit for emitting a plurality of light beams; and areduction afocal optical system for directing the light beams from thelight source unit toward the recording surface, the reduction afocaloptical system having an optical axis, wherein the reduction afocaloptical system comprises a spherical mirror, disposed on the opticalaxis, having a finite focal length and an equisolidangle projectionlens, disposed on the optical axis, having an optical characteristicsdefined by the following equation:

    hi'=2·f·sin(θi/2)

where hi' is a height of a light beam, leaving the equisolidangleprojection lens, taken from the optical axis or a height of an imagetaken from the optical axis, f is a focal length of the equisolidangleprojection lens and θi is an angle of incidence with respect to theequisolidangle projection lens; and wherein the focal point of theequisolidangle projection lens substantially coincides with that of thespherical mirror.

Accordingly, it is an object of the present invention to obtain acompact afocal optical system which satisfies hi'=m·hi where the heightsof incident and leaving light beams are hi and hi' and a magnificationis m and which is manufactured at low costs.

It is another object of the present invention to prevent a light beam tobe partially blocked in the afocal optical system.

It is a further object of the present invention to remove a noisecomponent from an incident light beam in the afocal optical system andto enhance telecentric characteristic of the afocal optical system.

It is another object of the present invention to obtain a multibeamrecording apparatus which is small despite an increased number ofchannels, which performs image drawing at a high accuracy, and which ismanufactured at low costs.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional afocal optical system;

FIG. 2 is a diagram showing other example of the conventional afocaloptical system;

FIG. 3 is a diagram of a conventional multibeam recording apparatus:

FIG. 4 is a plan view of an afocal optical system according to a firstpreferred embodiment of the present invention;

FIG. 5 is a plan view of an afocal optical system according to a secondpreferred embodiment of the present invention;

FIG. 6 is a plan view of an afocal optical system according to a thirdpreferred embodiment of the present invention;

FIG. 7 is a plan view of an afocal optical system according to a fourthpreferred embodiment of the present invention;

FIG. 8 is a plan view of an afocal optical system according to a fifthpreferred embodiment of the present invention;

FIG. 9 is a plan view of an afocal optical system according to a sixthpreferred embodiment of the present invention;

FIG. 10 is a plan view of a multibeam recording apparatus according to afirst preferred embodiment of the present invention;

FIG. 11 is a side view of a multibeam recording apparatus according to afirst preferred embodiment of the present invention;

FIG. 12 is a front view of a light source unit;

FIG. 13 is a plan view of a multibeam recording apparatus according to asecond preferred embodiment of the present invention;

FIG. 14 is a perspective view of a multibeam recording apparatusaccording to a fourth preferred embodiment of the present invention;

FIG. 15 is a plan view of a multibeam recording apparatus according to afourth preferred embodiment of the present invention;

FIG. 16 is a plan view showing how the light source parts are arranged;

FIG. 17 is a plan view showing a modification of the multibeam recordingapparatus according to the present invention;

FIG. 18 is a plan view showing another modification of the multibeamrecording apparatus according to the present invention;

FIG. 19 is a plan view showing a further modification of the multibeamrecording apparatus according to the present invention;

FIG. 20 is a diagram of a multibeam recording apparatus according to afifth preferred embodiment of the present invention;

FIG. 21 is a diagram of a multibeam recording apparatus according to asixth preferred embodiment of the present invention;

FIG. 22 is a diagram of a multibeam recording apparatus according to aseventh preferred embodiment of the present invention;

FIG. 23 is a diagram of a laser beam expander comprising the afocaloptical system of the present invention;

FIG. 24 is a diagram of an image input apparatus comprising the afocaloptical system of the present invention;

FIG. 25 is a diagram of a reduction projection apparatus comprising theafocal optical system of the present invention;

FIG. 26 is a diagram showing other example of the reduction projectionapparatus comprising the afocal optical system of the present invention;

FIG. 27 is a diagram of an expansion projector comprising the afocaloptical system of the present invention; and

FIG. 28 is a diagram of an illumination apparatus comprising the afocaloptical system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Afocal Optical System (1)First Preferred Embodiment

FIG. 4 is a plan view of an afocal optical system according to a firstpreferred embodiment of the present invention. The afocal optical system20A is comprised of a paraboroid mirror 22 and a stereographicprojection lens 24 which are disposed with a distance from each other sothat the focal points of the mirror 22 and the lens 24 coincide witheach other at a predetermined point A. Hence, a light beam LB1, enteringthe afocal optical system 20A parallel to the optical axis Z from alight source unit (described later) or an original, would be convertedinto a light beam LB3 leaving the afocal optical system 20A parallel tothe optical axis Z. In the afocal optical system 20A, the followingrelation

    hi'=m1·hi                                         (1)

is always satisfied where hi is a height of the incident light beam LB1taken from the optical axis Z and hi' is a height of the leaving lightbeam LB3 taken from the optical axis Z. The reason will be describedbelow.

For example, the light beam LB1 impinged onto the paraboroid mirror 22parallel to the optical axis Z at an object height (i.e., a height fromthe optical axis Z) hi is reflected by the paraboroid mirror 22 tobecome a reflected light beam LB2 which will pass through a point A,which is away from the paraboroid mirror 22 by the focal length f22, atan angle θi (FIG. 4). Here, because of the optical characteristics ofthe paraboroid mirror 22,

    tan (θi/2)=hi/(2·f22)                       (2)

The light beam LB2 passed through the point A then enters thestereographic projection lens 24 which has the following image heightcharacteristics ("stereographic projection characteristics" as hereintermed) that are given as:

    hi'=2·f24·tan (θi/2)               (3)

where f24 is a focal length of the stereographic projection lens 24 andhi' is an image height (i.e., a height from the optical axis Z). Hence,the image height hi' of the light beam LB3 emerging from thestereographic projection lens 24 is found by substituting Eq. 2 in Eq.3: ##EQU2## where m1 is a magnification of the afocal optical system20A.

Having such a construction, the afocal optical system promises excellentoptical performances but is less expensive to manufacture. The secreteis in that technique of forming the paraboroid mirror 22 is alreadycompleted to such an extent that the paraboroid mirror 22 is accuratelyformed in an easy manner at low costs. Further, the fact that theparaboroid mirror 22 includes only one surface which is to be processedand the paraboroid mirror 22 is large in aperture but small in F-numberalso contributes to reduction in size and manufacturing costs of theoptical system.

(2) Second Preferred Embodiment

FIG. 5 is a plan view of an afocal optical system according to a secondpreferred embodiment of the present invention. Provision of a paraboroidmirror 26 instead of the stereographic projection lens 24 is where theafocal optical system 20B of the second preferred embodiment (FIG. 5)stands different from the first preferred embodiment (FIG. 4). That is,the afocal optical system 20B is comprised of two paraboroid mirrors 22and 26 which have different focal lengths and which are disposed in afaced relation with each other in such a manner that their focal pointsmeet at a point A. A light beam LB1 entering the afocal optical system20B parallel to the optical axis Z is reflected by the paraboroid mirror22 and thereafter by the paraboroid mirror 26, whereby a light beam LB3emerges from the afocal optical system 20B parallel to the optical axisZ while the system 20B satisfies a condition similar to Eq. 4 (describedlater as Eq. 6). The mechanism of this will be described below.

For instance, when the light beam LB1 enters the afocal optical system20B parallel to the optical axis Z at an object height (i.e., a heightfrom the optical axis Z) hi, Eq. 2 is satisfied due to the opticalcharacteristics of the paraboroid mirror 22. Since the paraboroid mirror26 has the same characteristics (stereographic projectioncharacteristics) as those of the stereographic projection lens 24,

    hi'=2·f26·tan (θi/2)               (5)

where f26 is a focal length of the paraboroid mirror 26. Therefore, fromEqs. 2 and 5, ##EQU3## where m2 is a magnification of the afocal opticalsystem 20B. Hence, the effects promised in the first preferredembodiment are also promised in the second preferred embodiment. Inaddition, requiring the afocal optical system to be comprised of theonly two paraboroid mirrors 22 and 26, the second preferred embodimentproduces unique effect that chromatic aberration will not be created atall basically.

Although the first and the second preferred embodiments have beendescribed in relation to where the light beam LB1 enters from theparaboroid mirror 22 and the light beam LB3 goes out from the opticalelement (the stereographic projection lens 24 in the first preferredembodiment and the paraboroid mirror 26 in the second preferredembodiment) in the foregoing, the effects described above remain intactwhere the light beam LB1 enters from the optical element and the lightbeam LB3 goes out from the paraboroid mirror 22.

(3) Third Preferred Embodiment

FIG. 6 is plan view of an afocal optical system according to a thirdpreferred embodiment of the present invention. A major differencebetween the afocal optical system 20C and the first preferred embodiment(FIG. 4) is that while the first preferred embodiment uses the wholeparaboroid mirror 22, the third preferred embodiment uses a portion ofthe paraboroid mirror 22, more precisely, a region 22a which is off therotation symmetric axis of the paraboroid mirror 22 (In this preferredembodiment, the rotation symmetric axis coincides with the optical axisZ). In general, a paraboroid mirror which is formed by only the region22a is referred to as "off-axis paraboroid mirror."In sharp contrast, awhole paraboroid mirror (the paraboroid mirrors 22 and 26 of the firstand the second preferred embodiments) is herein referred to simply as"paraboroid mirror."

A further difference between the afocal optical system 20C and the firstpreferred embodiment is that a stop 28 is disposed at a point A wherethe focal points of the paraboroid mirror 22 and the stereographicprojection lens 24 coincide with each other. Since the other structuralfeatures are generally similar to those of the first preferredembodiment, redundant description will be omitted.

Similarly to the first preferred embodiment, when a light beam LB1impinges on the afocal optical system 20C of the third preferredembodiment parallel the optical axis Z, a light beam LB3 comes out fromthe afocal optical system 20C. Hence, the effects of the first preferredembodiment are attainable in the third preferred embodiment.

The third preferred embodiment creates still other effect because of theprovision of the off-axis paraboroid mirror 22a. More particularly, inthe afocal optical system 20A of the first preferred embodiment, asshown in FIG. 4, a portion of the incident light beam LB1 is blocked bythe stereographic projection lens 24 so that a center portion of thelight beam LB3 leaving the afocal optical system 20A is blocked. Thisproblem also occurs where the light beam LB1 enters the optical systemfrom the stereographic projection lens 24. In sharp contrast, in thethird preferred embodiment, as shown in FIG. 6, the light beam LB3leaves the afocal optical system 20C without partially blocked by thestereographic projection lens 24.

Further, in the third preferred embodiment, since the stop 28 isdisposed at the point A where the focal points of the off-axisparaboroid mirror 22a and the stereographic projection lens 24 coincidewith each other, a noise component is removed from the light beam LB1and the afocal optical system 20C has an improved telecentric quality inthe imaging side. These effects will be described in detail later.

(4) Fourth Preferred Embodiment

FIG. 7 is a plan view of an afocal optical system according to a fourthpreferred embodiment of the present invention. The afocal optical system20D is comprised of an off-axis paraboroid mirror 22a and a paraboroidmirror 26 which are disposed in a faced relation in such a manner thattheir focal points coincide with each other at a point A. In otherwords, the afocal optical system 20D is the same as the system of thesecond preferred embodiment (FIG. 5) except that the off-axis paraboroidmirror 22a is used in stead of the paraboroid mirror 22 and that a stop28 is disposed at the point A. Hence, the afocal optical system 20D notonly attains effects similar to those attainable in the second preferredembodiment but also produces the effects which are described immediatelyabove in relation to the provision of the off-axis paraboroid mirror 22aand the stop 28.

(5) Fifth Preferred Embodiment

It is to be noted that the paraboroid mirror 26 may be replaced with anoff-axis paraboroid mirror 26a as shown in FIG. 8. Such an afocaloptical system 20E, which is comprised of two off-axis paraboroidmirrors 22a and 26a, also promises the same effects as those attainablein the fourth preferred embodiment.

(6) Sixth Preferred Embodiment and Other Embodiments

FIG. 9 is a plan view of an afocal optical system according to a sixthpreferred embodiment of the present invention. The afocal optical system20F is comprised of a spherical mirror 28 and an equisolidangleprojection lens 29. In the afocal system 20F, the focal points of thespherical mirror 28 and the equisolidangle projection lens 29 coincidewith each other at a point C, the effects hereinabove described areensured. The reason is as follows.

When laser beams LB1 enters the spherical mirror 28 parallel to theoptical axis Z, spherical aberration occurs at the spherical mirror 28because of the optical characteristics which are inherent to a sphericalmirror. Here, assume that the height of the incident laser beams LB1taken from the optical axis Z is hi and laser beams LB2 reflected by thespherical mirror 28 crosses the optical axis Z at an angle θi, thecharacteristics of the spherical mirror 28 cause that the laser beamsLB2 satisfy:

    sin (θi/2)=hi/(2·f28)                       (8)

where f28 is a focal length of the spherical mirror 28. On the otherhand, the equisolidangle projection lens 29, formed by three lenses L12to L14, for example, exhibits an optical characteristic that is givenas:

    hi'=2·f29·sin (θi/2)               (9)

where hi' is a height of a laser beam coming from the equisolidangleprojection lens 29. Substituting Eq. 8 in Eq. 9, ##EQU4## Thus, sinceEq. 10 is satisfied by combining the spherical mirror 28 and theequisolidangle projection lens 29 and since the equisolidangleprojection lens 29 causes aberration that is opposite to the aberrationwhich is caused by the spherical mirror 28 so that aberrations cancelout each other in the afocal optical system 20F as a whole, excellentoptical characteristics are promised.

In the foregoing, the third to the fifth preferred embodiments have beendescribed in relation to where the light beam LB1 enters from theoptical element (the stereographic projection lens 24 in the thirdpreferred embodiment; the paraboroid mirror 26 in the fourth preferredembodiment; the off-axis paraboroid mirror 26a in the fifth preferredembodiment) and the light beam LB3 leaves the optical system from theoff-axis paraboroid mirror 22a. The effects of these preferredembodiments will not be punctured where the light beam LB1 enters theoff-axis paraboroid mirror 22a and the light beam LB3 leaves from theoptical element.

In the first and the second preferred embodiment, the stop 28 may bedisposed at the point A, in which case the effects above (elimination ofnoise, etc) are attainable. Conversely, the stop 28 is not an essentialelement in the afocal optical systems of the third to the fifthpreferred embodiments.

B. Optical Apparatus Comprising Afocal Optical System

In the following, optical apparatuses comprising the afocal opticalsystems 20A to 20E will be described.

B-1. Multibeam Recording Apparatus (1) First Preferred Embodiment

FIGS. 10 and 11 are a plan view and a side view, respectively, of amultibeam recording apparatus according to a first preferred embodimentof the present invention. The multibeam recording apparatus comprises alight source unit 10 for emitting a plurality of laser beams, areduction afocal optical system 20A, an afocal optical system 30 and arotation cylinder 40. In synchronism with rotation of the rotationcylinder 40 with a photosensitive material FM wound therearound in aprimary scanning direction X, light beams from the light source unit 10move in a sub scanning direction Y, which is approximately perpendicularto the primary scanning direction, through the reduction afocal opticalsystem 20A and the afocal optical system 30. As a result, a desiredimage is recorded on the photosensitive material FM.

FIG. 12 is a front view of the light source unit 10. In FIG. 12, thelight source unit 10 is comprised of a plurality of light source parts12 which are arranged at predetermined pitches Pa. Each light sourcepart 12 is formed by a semiconductor laser 14 and a collimating lens 16.A light beam from the semiconductor laser 14 is collimated by thecollimating lens 16 to become a parallel light beam which will be thenemitted from an aperture 18 of the light source part 12 parallel to theoptical axis Z (FIGS. 10 and 11). The aperture 18 is disposed on thefocal plane of a paraboroid mirror 22. As understood from FIG. 12, thelight source parts 12 are arranged so as to partially overlap with eachother in the primary scanning direction X. This is to prevent a split inscanning lines, that is, separation of adjacent scanning lines from eachother due to mechanical dimensional restraints of the light source parts12. In addition, to avoid mechanical interference with the reductionafocal optical system 20A, the light source parts 12 are divided intotwo groups in their arrangement (FIG. 11).

The reduction afocal optical system 20A has the same structure as thatof the conventional afocal optical system of FIG. 4. That is, thereduction afocal optical system 20A is comprised of the paraboroidmirror 22 and a stereographic projection lens 24 which are disposed insuch a manner that the focal points of the mirror 22 and the lens 24coincide with each other at a predetermined point A. Hence, a laser beamLB1 entering the reduction afocal optical system 20A parallel to theoptical axis Z from the light source unit 10 would converted into alaser beam LB3 leaving the reduction afocal optical system 20A parallelto the optical axis Z. Further, in the reduction afocal optical system20A, the Eq. 1 is always satisfied, and hence the laser beams LB1emitted from the light source unit 10 at the same pitches Pa are focusedat a rear focal plane FP1 of the stereographic projection lens 24 sothat intermediate images of the apertures 18 are formed on the rearfocal plane FP1 at equal intervals. It is to be noted that the apertureimages are imaged on the rear focal plane FP1 only when the apertures 18are arranged on the focal plane of the paraboroid mirror 22. If theapertures 18 are off the focal plane of the paraboroid mirror 22, theaperture images, too, will be off the rear focal plane FP1. Thedisplacement of the aperture images from the rear focal plane FP1 isdetermined by a longitudinal magnification of the reduction afocaloptical system 20A.

As shown in FIGS. 10 and 11, the afocal optical system 30 is disposedbetween the reduction afocal optical system 20A and the rotationcylinder 40. The afocal optical system 30 is comprised of a zoom lens 32which is formed by lenses L4 to L9 and an afocal optical system 34 whichis formed by lenses L10 and L11. The zoom lens 32 has afocalcharacteristics or telecentric characteristics, and the magnificationration thereof can be varied by moving at least one of the lenses L4 toL9 while the distance between an object and an image is kept constant.In the afocal optical system 30, the image plane of the zoom lens 32coincides with a front focal plane of the lens L10 of the afocal opticalsystem 34 at a plane FP2 so that the zoom lens 32 and the afocal opticalsystem 34 as a whole are also afocal. The object plane of the zoom lens32 coincides with the rear focal plane of the stereographic projectionlens 24 of the reduction afocal optical system 20A at the plane FP1while the photosensitive material FM (recording surface) is placed atthe rear focal plane of the lens L11 of the afocal optical system 34.Hence, the intermediate images (i.e., the aperture images) formed on theplane FP1 are reduced at an appropriate magnification by the afocaloptical system 30 and imaged as the aperture images (i.e., beam spots)on the photosensitive material FM. Therefore, the beam spots on thephotosensitive material FM are arranged at equal intervals, that is, thebeam pitches are uniform. It is to be noted that the beam spots areformed on the photosensitive material FM only when the intermediateimages are formed on the plane FP2. If the intermediate images are offthe plane FP2, as described above with respect to the reduction afocaloptical system 20A, the beam spots will be off the photosensitivematerial FM. The displacement of the beam spots from the photosensitivematerial FM is determined by a longitudinal magnification of the afocaloptical system 34.

As described above, in this embodiment, the plurality of the laser beamsLB1 from the light source unit 10 are converged at a point by theparaboroid mirror 22, and the laser beams from that point are emittedfrom the reduction afocal optical system 20A through the stereographicprojection lens 24. Hence, the reduction afocal optical system 20Aremains small even when the number of channels is increased. That is,technique of forming the paraboroid mirror 22 is already completed tosuch an extent that the paraboroid mirror 22 is accurately formed in aneasy manner at low costs. Further, the fact that the paraboroid mirror22 includes only one surface which is to be processed and the paraboroidmirror 22 is large in diameter but small in F-number also contributes toreduction in size and manufacturing costs of the optical system.Further, since the laser beams LB3 leave the reduction afocal opticalsystem 20A parallel to the optical axis Z while the system 20A satisfiesEq. 1, the beam pitches on the photosensitive material FM (recordingsurface) are uniform, enabling highly accurate image drawing. Stillfurther, since each laser beam is irradiated onto the photosensitivematerial FM from approximate upright as shown in FIGS. 10 and 11 in thisembodiment, even if the photosensitive material FM is moved along theoptical axis Z, no change in the magnification and hence accurate imagedrawing on the photosensitive material FM are promised.

(2) Second Preferred Embodiment

FIG. 13 is a plan view of a multibeam recording apparatus according to asecond preferred embodiment of the present invention. A major differencebetween the illustrative multibeam recording apparatus and the firstpreferred embodiment is that provision of a paraboroid mirror 26 insteadof the stereographic projection lens 24. That is, the multibeamrecording apparatus comprises the afocal optical system 20B of FIG. 5 asa reduction afocal optical system. In more detail, the reduction afocaloptical system 20B is comprised of the two paraboroid mirrors 22 and 26which are disposed in a faced relation in such a manner that their focalpoints coincide with each other at a point A and that laser beams fromthe light source unit 10 are serially reflected by the paraboroidmirrors 22 and 26 to be thereafter irradiated onto the photosensitivematerial FM through the afocal optical system 30 which has the samestructure as the corresponding optical system of the first preferredembodiment.

The focal length f22 of the paraboroid mirror 22 is different from thefocal length f26 of the paraboroid mirror 26. Where the paraboroidmirror 26 is provided to replace the stereographic projection lens 24,the laser beams LB1 parallel to the optical axis Z impinging on thereduction afocal optical system 20B would result in that the laser beamsLB3 parallel to the optical axis Z leave the reduction afocal opticalsystem 20B while satisfying Eq. 1, which is similar to the firstpreferred embodiment. The reason is as follows.

(3) Third Preferred Embodiment

Although the foregoing has described that the afocal optical systems 20Aand 20B are formed by the paraboroid mirror 22 and the stereographicprojection lens 24 (first preferred embodiment) or formed by the twoparaboroid mirrors 22 and 26 (second preferred embodiment), it ispossible that the afocal optical system is formed by the sphericalmirror 28 and the equisolidangle projection lens 29 of FIG. 9.

The third preferred embodiment is generally identical to the first andthe second preferred embodiments except for the afocal optical system,and therefore, similar description will be simply omitted.

(4) Fourth Preferred Embodiment

FIGS. 14 and 15 are a perspective view and a plan view, respectively, ofa multibeam recording apparatus according to a fourth preferredembodiment of the present invention. The illustrative multibeamrecording apparatus largely departs from the first preferred embodimenton two points. The first major difference is that the afocal opticalsystem 20C of FIG. 6 is used as a reduction afocal optical system in thefourth embodiment. The second major difference is that the optical axisZ1 of the afocal optical system 20C is displaced from the optical axisZ2 of the afocal optical system 30 by a predetermined distance ΔY in thesub scanning direction Y. Although no practical problem will occur evenif the two optical axes coincide with each other as in the firstpreferred embodiment (FIG. 10), when the off-axis paraboroid mirror 22ais used, some of the laser beams LB3 from the afocal optical system 20Cwill not enter the afocal optical system 30. In sharp contrast, in thefourth preferred embodiment, since the optical axes are not aligned toeach other, the whole afocal optical system 30 is involved in directingthe laser beams LB3 toward the photosensitive material FM from theafocal optical system 21)C as shown in FIG. 15. Hence, a compact afocaloptical system 30 is obtained.

In FIG. 14, indicated at reference numeral 50 is a base which is freelyslidable in the sub scanning direction Y. The base 50 mounts the lightsource unit 10, the off-axis paraboroid mirror 22a, the stereographicprojection lens 24, a reflecting mirror 52, the zoom lens 32, areflecting mirror 54 and an afocal optical system 34 thereon, therebyforming a recording head. The afocal optical system 34 is comprised of alens 10 and lenses L15 to L17. The recording head is provided with adrive mechanism (not shown) which moves the recording head in the subscanning direction Y. To adjust the magnification of the zoom lens 32,the zoom lens 32 is linked to a pulse motor 56.

FIG. 16 is a plan view showing the arrangement of the light source parts12. In FIG. 16, a plurality of light source parts 12 aretwo-dimensionally arranged at the same pitches Pa in the light sourceunit 10. Similarly to the first preferred embodiment, the light sourceparts 12 are displaced from each other by (the scanning pitch Ps)/(themagnification M of the optical system) to partially overlap with eachother in the primary scanning direction X in order to prevent a split inscanning lines.

In the multibeam recording apparatus having such a construction, aplurality of laser beams LB1 from the light source unit 10 parallel tothe optical axis Z1 are reflected by the off-axis paraboroid mirror 22aand then focused on the rear focal plane FP1 of the stereographicprojection lens 24 therethrough. Since the apertures 18 are arranged onthe focal plane of the off-axis paraboroid mirror 22a, intermediateimages (i.e., the aperture images) are formed on the plane FP1. Theimage height of each intermediate image and the height of each laserbeam LB1 from the light source part 12 satisfy Eq. 1 as describedearlier in relation to the first preferred embodiment, and therefore, aplurality of intermediate images are formed on the plane FP1 at equalintervals. The intermediate images are then reduced at an appropriatemagnification by the afocal optical system 30 and formed on thephotosensitive material FM (recording surface) which is wound around therotation cylinder 40 as images (beam spots).

As described above, in the fourth preferred embodiment, similarly to thefirst preferred embodiment, the laser beams LB1 from the light sourceunit 10 are imaged as intermediate images on the plane FP1 through theparaboroid mirror 22 and the stereographic projection lens 24 while theembodiment satisfies Eq. 1, and then imaged on the photosensitivematerial FM by the afocal optical system 30. Hence, the effectsattainable in the first preferred embodiment are also attainable in thefourth preferred embodiment.

Although the fourth preferred embodiment is related to where theapertures 18 are used, the apertures 18 are not essential elements. Anexample of where the apertures are not provided is shown in FIG. 17 inwhich the laser beams LB1 from the semiconductor lasers 14 arecollimated by the collimating lenses 16 into parallel light beams whichwill be then reflected by the paraboroid mirror 22 and converged as beamwaists at the focal point D of the paraboroid mirror 22. Since the focalpoint D of the paraboroid mirror 22 is the front focal point of thestereographic projection lens 24, the beam waists of the laser beams LB3passed through the stereographic projection lens 24 is formed at therear focal plane FP1 of the stereographic projection lens 24. In asimilar manner, beam waists are formed on the photosensitive material FMwhich is disposed at the rear focal plane of the lens L17 of the afocaloptical system 34 (FIG. 15). Hence, images are drawn with extremelysmall beam spots at as high accuracy as that of where the apertures areprovided.

The afocal optical system 30 is not essential to the multibeam recordingapparatus. As shown in FIG. 18, the multibeam recording apparatus may beformed only by the light source unit 10 and the reduction afocal opticalsystem 20C, in which case, the photosensitive material FM (recordingsurface) is to be disposed at the rear focal plane FP1 of thestereographic projection lens 24.

The afocal optical system 30 may be formed by only the zoom lens 32 asshown in FIG. 19. In this case, the photosensitive material FM must bedisposed at the image plane FP2 of the zoom lens 32.

Although the preceding embodiments require that the image plane of thezoom lens (afocal system) 32 coincides with the front focal plane of thelens L10 of the afocal optical system 34 to form the optical system 30so that the optical system 30 becomes afocal, the respective opticalsystem forming the afocal optical system 30 (the lens 32 in the firstpreferred embodiment and the optical system 34 in the second preferredembodiment) needs not be afocal. That is, it is only necessary that theoptical system 30 is afocal as a whole.

(5) Fifth Preferred Embodiment

FIG. 20 is a diagram of a multibeam recording apparatus according to afifth preferred embodiment of the present invention. The fifth preferredembodiment is different from the first preferred embodiment regardingthe light source unit 10 but is otherwise generally the same as thefirst preferred embodiment. Hence, only the light source unit 10 will bedescribed in terms of structure and the other structures will beomitted.

In the light source unit 10, a solid laser 62 is used as a laser beamsource instead of the semiconductor laser 12. One laser beam LB4 fromthe solid laser beam 62 is allowed into a beam splitter 64 where it isdivided into a plurality of laser beams. The divided laser beams entersa multi-channel modulator 66 where they are modulated each in accordancewith an image signal. The laser beams are then omitted from the lightsource unit 10 toward the reduction afocal optical system 20C. A gas andother suitable laser may replace the solid laser 62.

In this embodiment, as shown in FIG. 20, beam waists BW are formed at aposition which corresponds to where the apertures 18 of the firstpreferred embodiment are located. Hence, the beam waists of the dividedlaser beams are located at the planes FP1 and FP2 and on thephotosensitive material FM (recording surface), promising a sharp imageto be recorded on the photosensitive material FM. Of course, the effectsof the fourth preferred embodiment remain the same since the reductionafocal optical system 20C and the optical system 30 remain unchangedregarding structure.

When the beam splitter 64 is used, the beam waists of the divided laserbeams are formed at different positions in different channels, and thedifferences of the waist-forming positions are determined by a verticalmagnification of the optical system as a whole. Since this type ofrecording apparatus in most cases has a large reduction to perform highdensity image drawing, however, the differences of the waist-formingpositions are extremely small and therefore negligible in practical use.

(6) Sixth Preferred Embodiment

Although the foregoing has described such an apparatus for recording animage on the photosensitive material FM which is wound around therotation cylinder 40; the present invention is applicable to anapparatus which records an image on a photosensitive material FM whichis carried on the inner surface of a cylinder.

FIG. 21 is a perspective view of a scanning part of this type ofmultibeam recording apparatus. In FIG. 21, a photosensitive material FMis carried by the inner surface of a holder 72 which resembles a hollowcylinder which is divided parallel to its axis. On the center line ofcurvature CL of the holder 72, a rotation mirror 74 having a reflectivesurface which is parallel to the center line CL is supported by a pairof frames 76 and 76 for free rotation. Connected to a motor 80 via abelt 78, the rotation mirror 74 rotates when driven by the motor 80.Below the rotation mirror 74, a stationary mirror 81 is fixed to theframes 76 and 76.

A ball screw 82 runs across the frames 76 and 76 and a beam head 84 isengaged with the ball screw 82. When a motor 86 rotates which is linkedto an end of the ball screw 82, the beam head 84 slides in the subscanning direction Y while guided by guides 88 and 88.

A rectangular prism 90 is mounted on the beam head 84. A plurality oflaser beams LB originating from the light source unit 11) strike,through the reduction afocal optical system 20A and the afocal opticalsystem 30, the rectangular prism 90 where they are reflected toward aconverging lens 92 which is mounted on the beam head 84. Laser beamsfrom the converging lens 92 are irradiated on the photosensitivematerial FM through the station mirror 78 and the rotation mirror 74.

Thus, when laser beams from the afocal optical system 30 are scanned inthe primary scanning direction X by driving the motor 80 and rotatingthe rotation mirror 74 while the beam head 84 is moved in the subscanning direction Y by driving the motor 86, a desired image isrecorded on the photosensitive material FM.

(7) Seventh Preferred Embodiment

FIG. 22 is a diagram of a multibeam recording apparatus according to aseventh preferred embodiment of the present invention. The multibeamrecording apparatus comprises the light source unit 10 for emitting twolaser beams, the reduction afocal optical system 20C which is formed bythe off-axis paraboroid mirror 22a and the stereographic projection lens24, the afocal optical system 30 which is formed by two lenses L18 andL19, and an XY stage 100 for mounting and two-dimensionally registeringa target object 122.

In the light source unit 10, a laser beam LB5 from an argon laser 110impinges on a beam splitter 116 through a shutter 112 and a reflectingmirror 114. Some components of the laser beam LB5 are reflected by thebeam splitter 116 and the remaining components of the laser beam LB5 aretransmitted by the beam splitter 116 and reflected by a mirror 118.Thus, in the light source unit 10, one laser beam LB5 is divided intotwo parallel laser beams LB6 an LB7 which will be emitted toward thereduction afocal optical system 20C. Although not shown in FIG. 22, thebeam splitter 116 and the mirror 118 are linked to a drive mechanism sothat the beam splitter 116 and the mirror 118 are individually movablein a direction Z in which the laser beam LB5 propagates. By adjusting adistance between the beam splitter 116 and the mirror 118, the beampitch Pa of the laser beams LB6 and LB7 from the light source unit 10 ischanged.

The laser beams LB6 and LB7 from the light source unit 10 are focusedthrough the reduction afocal optical system 20C as intermediate imagesat a predetermined position and then reflected by a reflection mirror120 to be advanced to the afocal optical system 30. The intermediateimages are then reduced by the afocal optical system 30 at a propermagnification and imaged on the target object 122 which is placed on theXY stage 100. The multibeam recording apparatus is capable of recordingimages on, for example, regions 122a and 122b of the target object 122at a predetermined pitch at the same time. Although the foregoing hasdescribed that the preceding embodiments use a laser such as thesemiconductor laser 14 and the solid laser 62 as a light source foremitting a light beam, an LED may be used as such.

B-2. Laser Beam Expander

FIG. 23 is a diagram of a laser beam expander comprising the afocaloptical system 20A of FIG. 4. In FIG. 23, the laser beam expander 50 isformed by the afocal optical system 20A and a reflecting mirror 54 whichhas an aperture 52 in the center. A light beam LB1 from a light source(not shown) travelling parallel to the optical axis Z passes through theaperture 52 of the mirror 54 and enters the afocal optical system 20A.As a result, a light beam LB3 comes out of the afocal optical system 20Aparallel to the optical axis Z while satisfying Eq. 4 as describedearlier. The light beam LB3 is then reflected by the reflecting mirror54 which is oriented at a certain angle (e.g., 45 degrees) to theoptical axis Z and directed perpendicular to the direction of theincident light. Hence, the beam diameter of the light beam LB3 isexpanded in accordance with the magnification of the afocal opticalsystem 20A (=f22/f24; f22>f24). Thus, a laser beam expander is obtainedwhich produces a light beam of a larger diameter without inviting theproblems which are inherent in the conventional technique (FIG. 1).

In addition, in the embodiment shown in FIG. 23, a stop 28 is interposedat the point A where the focal points of the paraboroid mirror 22 andthe stereographic projection lens 24 coincide with each other. Thoughnot essential to the laser beam expander 50, disposed at the point Awhich corresponds to the entrance pupil of the stereographic projectionlens 24, the stop 28 serves as a spatial filter. The intensity of lightbeam LB1 applied to the laser beam expander 50 usually has a Gaussiandistribution. In some cases, a noise component is included in the lightbeam LB1. Disposed at the focal point of the stereographic projectionlens 24, the stop 28 cuts noise component.

Although the laser beam expander of FIG. 23 comprises the afocal opticalsystem 20A, any one of the afocal optical systems 20B to 20E describedthe above may be used instead.

The optical apparatus above serves as a laser beam compressor ifconstructed to have an opposite optical path in which the light beam LB1impinges in an opposite manner.

B-3. Image Input Apparatus

FIG. 24 is a diagram of an image input apparatus comprising the afocaloptical system 20C of FIG. 6. The image input apparatus 60 comprises alamp 66 for illuminating an original 64 which is placed on a table 62.The original 64 is approximately evenly illuminated with light from the,lamp 66.

Comprising the afocal optical system 20C of FIG. 6, the image inputapparatus 60 is telecentric on both the object side (original 64 side)and the image side (image input &vice side), i.e., a both-sidetelecentric optical system. When the lamp 66 is turned on, a light beamLB1 originating from the original 64 impinges on the off-axis paraboroidmirror 22a. A light beam LB2 reflected by the off-axis paraboroid mirror22a passes through the stop 28 to be allowed to the stereographicprojection lens 24 which images the light beam LB2 on an image inputdevice 68 such as CCD element. The image and other information of theoriginal 64 are thus read by the image input device 68.

In this embodiment, it is the stop 28 that enhances the telecentricquality of the afocal optical system 20C. That is, due to the stop 28,principal rays PR of the light from the original 64 cross the opticalaxis Z at the point A, or the entrance pupil of the stereographicprojection lens 24, whereby the principal rays PR included in the lightbeam LB3 from the stereographic projection lens 24 all become parallelto the optical axis Z, i.e., perpendicular to the image input device 68at every image height. In addition, since the stop position A is thefocal point of the off-axis paraboroid mirror 22a, principal rays PRincluded in the light beam LB1 from the original 64 become perpendicularto the surface of the original and parallel to the optical axis Z at anyposition on the surface of the original.

As described above, the image input apparats 60, comprising the afocaloptical system 20C (though the light propagates in an opposite directionto that shown in FIG. 6), images light from the original 64 on the imageinput device 68 while always satisfying Eq. 7. In addition, the imageinput apparats 60 is telecentric on both the image and the object sides.Since this allows that the data of the original is inputted in atelecentric condition, even if there is an inconvenience regarding theoriginal 64, e.g., the original 64 rises from the table 62, adimensional error would not be easily created. In addition, although theimage input device 68 is equipped with an optical window (not shown) inmost cases, since the image input apparats 60 is telecentric on theimage side (image input device 68 side), only light from uprightdirection is allowed into the image input device 68, thereby preventingaberration which is created by light incident upon the optical window atan angle and hence improving the efficiency of data input of theoriginal.

The afocal optical system 20C may be replaced with any one of the afocaloptical systems 20A, 20B, 20D and 20E of the other preferredembodiments.

B-4. Reduction Projection Apparatus

FIG. 25 is a diagram of a reduction projection apparatus comprising theafocal optical system 20A of FIG. 4. The reduction projection apparatus70 is an apparatus for reducing the images of reticles 76 andtransferring the images once at a time onto a resist film 74 which isformed on a silicon substrate 72. The reduction projection apparatus 70is comprised of illumination optical systems 80 for irradiating thereticles 76 from the back surfaces of the reticles 76, the afocaloptical system 20A of FIG. 4, and a stage 82 for mounting the siliconsubstrate 72.

In each illumination optical system 80, a light beam from a lamp 86which is equipped with an elliptical mirror 84 impinges on a fly-eyelens 90 through a cold mirror 88. A light beam from the fly-eye lens 90enters a collimating lens 92 where it is collimated to become a parallellight beam which will be then reflected by a mirror 94 back onto theback surfaces of the reticles 76. A light beam transmitted by thereticles 76 is imaged on the resist film 74 by the afocal optical system20A. The images of the reticles are thus transferred onto the resistfilm 74.

As described above, the images of the reticles are transferred onto theresist film 74 using the afocal optical system 20A which is telecentricon both the image and the object sides. Hence, even though the reticles76 and the resist film 74 include a deficiency such as a partially risenportion or a warped portion, the reticle images are transferred at arelatively good accuracy.

If resolution is not an issue, the optical system may be telecentriconly on the object side (reticle 76 side) as shown in FIG. 26 as far asthe height hi' at the resist film 74 and the height hi on the objectside satisfy Eq. 4. In such a case, since the optical system is nottelecentric on the imaging side (resist film 74 side), it is possible totransfer larger reticle images onto the resist film 74 if at a littledegraded transfer accuracy when the reticles and the resist film includea deficiency such as a partially risen portion and a warped portion.

Although the foregoing is related to where the images of the tworeticles 76 are transferred onto the resist film 74 once at a time,three or more reticles 76 may be transferred at the same time. Further,instead of the afocal optical system 20A, any one of the afocal opticalsystems 20C, 20D and 20E of the third, the fourth and the fifthpreferred embodiments to transfer the image of one reticle 76.

B-5. Expansion Projector

FIG. 27 is a diagram of an expansion projector comprising the afocaloptical system 20C of the third preferred embodiment. Constructed as anapparatus for expanding the image of a target object 102 and projectingthe image on a screen 104, the expansion projector 100 comprises a lamp106 for illuminating the target object 102, a stage 108 and the afocaloptical system 20C.

The afocal optical system 20C is similar to the afocal optical system ofFIG. 6 except for provision of a reflecting mirror 110. Hence, when thelamp 106 is turned on, a light beam from the target object 102 is imagedon the back surface of the screen 104 through the afocal optical system20C, allowing that the expanded image of the target object 102 isobserved from the front surface of the screen 104.

Since also in this embodiment a stop 28 is disposed at the point A wherethe focal points of the paraboroid mirror 22 and the stereographicprojection lens 24 coincide with each other, principal rays included ina light beam from the target object 102 advance perpendicular to thetarget object 102 and principal rays included in a light beam from theoff-axis paraboroid mirror 22a advance perpendicular to the screen 104,i.e., the expansion projector is telecentric on both the image and theobject sides, the reason being the same as that described before. Hence,the expanded image of the target object 102 is observed at a highresolution.

B-6. Illumination Apparatus

FIG. 28 is a diagram of an illumination apparatus comprising the afocaloptical system 20E. The illumination apparatus 120, intended to be usedin a proximity exposure apparatus, comprises a light source unit 122 andthe afocal optical system 20E which is formed by two off-axis paraboroidmirrors 22a and 26a.

The light source unit 122 includes a lamp 126 which has an ellipticalmirror 124. A light beam from the lamp 126 impinges on a fly-eye lens130 through a cold mirror 128. A light beam from the fly-eye lens 130enters a collimating lens 132 where it is collimated into a parallellight beam LB1. Hence, an imaginary plane B immediately below thecollimating lens 132 is irradiated by the parallel light beam which haseven intensity distribution.

Since the afocal optical system 20E is identical to the afocal opticalsystem of FIG. 8 described earlier, the parallel light beam LB1 from thelight source unit 122 is expanded by the afocal optical system 20E whichsatisfies:

    hi'=m5·hi

where m5=f22a/f26a; f22a is a focal length of the off-axis paraboroidmirror 22a; f26a is a focal length of the off-axis paraboroid mirror26a; and m5 is a magnification of the afocal optical system 20E.Expanded in such a manner, the parallel light beam LB1 is irradiatedparallel to the optical axis Z onto a surface-to-be-illuminated (e.g., amask plate 138 placed immediately above a resist film 136 disposed on aglass 134 if the illumination apparatus 120 is used in a proximityexposure apparatus). In FIG. 28, indicated at numerical reference 140 isa table for mounting the glass 134.

As described above, since the illumination apparatus 120 uses the afocaloptical system 20E which is comprised of the two off-axis paraboroidmirrors 22a and 26a, light is irradiated onto thesurface-to-be-illuminated perpendicular thereto with uniformillumination distribution. Further, by properly combining the focallengths of the off-axis paraboroid mirrors 22a and 26a and changing themagnification m5 of the afocal optical system 20E, an illumination areais adjusted as desired.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. An afocal optical system having an optical axis,comprising:a first paraboroid mirror, disposed on the optical axis,having a finite focal length; and an optical element, disposed on theoptical axis, having a stereographic projection characteristics which isdefined by the following equation:

    hi'=2·f·tan(θi/2)

where hi' is a height of a light beam, leaving said optical element,taken from the optical axis or a heigh of an image taken from theoptical axis, f is a focal length of said optical element and θi is anangle of incidence with respect to said optical element, the focal pointof said optical element substantially coinciding with that of said firstparaboroid mirror.
 2. An afocal optical system of claim 1, wherein saidoptical element is a second paraboroid mirror.
 3. An afocal opticalsystem of claim 2, wherein at least one of said first and secondparaboroid mirrors is an off-axis paraboroid mirror.
 4. An afocaloptical system of claim 1, wherein said optical element is astereographic projection lens.
 5. An afocal optical system of claim 4,wherein said first paraboroid mirror is a off-axis paraboroid mirror. 6.An afocal optical system of claim 1, further comprising an aperture stopdisposed at a point where the focal points of said first paraboroidmirror and said optical element coincide with each other.
 7. A multibeamrecording apparatus for recording an image on a recording surface,comprising:a light source unit for emitting a plurality of light beams;and a reduction afocal optical system for directing said light beamsfrom said light source unit toward said recording surface, saidreduction afocal optical system having an optical axis, wherein saidreduction afocal optical system comprises a first paraboroid mirror,disposed on the optical axis, having a first finite focal length; and anoptical element, disposed on the optical axis, having a stereographicprojection characteristics which is defined by the following equation:

    hi'=2·f·tan(θi/2)

where hi' is a height of a light beam, leaving said optical element,taken from the optical axis or a height of an image taken from theoptical axis, f is a second finite focal length of said optical elementand θi is an angle of incidence with respect to said optical element;and wherein the focal point of said optical element substantiallycoincides with that of said first paraboroid mirror.
 8. A multibeamrecording apparatus of claim 7, wherein said optical element is a secondparaboroid mirror, and wherein the first and second finite focal lengthsare different from each other.
 9. A multibeam recording apparatus ofclaim 7, wherein said optical element is a stereographic projectionlens, and wherein the first and second finite focal lengths aredifferent from each other.
 10. A multibeam recording apparatus of claim7, further comprising an afocal optical system having an optical axisdisposed between said reduction afocal optical system and said recordingsurface, wherein the optical axis of said afocal optical system isparallel to and displaced from the optical axis of said reduction afocaloptical system.